Model design of a compact delayed gamma-ray moderator system using ²⁵²Cf for safeguards verification measurements

Delayed gamma-ray spectroscopy (DGS) is an active-interrogation nondestructive assay technique that can be used to determine the composition of a nuclear material sample by comparing the ratios of fission product gamma-ray peak intensities. However, high-radioactivity nuclear material (HRNM) contains long-lived fission products that can overwhelm a detector and shielding must be used to reduce the count rate, while minimally affecting those gamma rays from short-lived fission products with energy above 3-MeV. To compensate for the signal loss through the shielding, low-energy neutrons are required to induce more fission events from the high thermal cross-section of fissile nuclides. To improve practical safeguards DGS capabilities, we are developing a compact interrogation system to moderate $\sim$ 2-MeV neutrons that are easier to moderate than 14-MeV neutrons from standard deuterium-tritium generators. This work describes the optimization of an ideal moderator system for a ²⁵²Cf neutron point source that results in a neutron fluence of $25.9\times {10}^{-4}$ cm⁻² $n_{\mathrm{source}}^{-1}$ passing through the sample space with $\gtrsim$ 70% of those below 1-eV. Modifications for practical fabrication resulted in $⩽20$% reductions of the flux compared to the optimized ideal design. Finally, evaluations made of HRNM DGS signals and backgrounds conclude that a ²⁵²Cf source intensity of $8.9\times {10}^7$ neutrons per second is required for a single-pass interrogation within this ideal moderator system. However, this can be as low as $3.6\times {10}^6$ neutrons per second using smaller samples that require less shielding.